ENH : add flag to box_plot and bxp to manage (or not) xticks

Closes #2921
 - added test for managa_xtick option to boxplot
 - added CHANGELOG and whats_new.rst entry for manage_xticks
TST : fixed duplicate test name
Merge pull request #2922 from tacaswell/boxplot_manage_xlim

ENH : add flag to box_plot and bxp to manage (or not) xticks